We developed a reproducible assay for the measurement of concanavalin A (Con A)-inducible suppressor cells. Using this assay system we studied suppressor cell activity of peripheral blood lymphocytes (PBL) from urologic cancer and control patients. Although lymphocyte proliferation to Con A was depressed in many of the cancer patients, no difference in suppressor cell activity between cancer and control patients was observed. These results suggest that suppressor cells of the type induced by Con A are not responsible for the immunosuppression observed in urologic cancer patients. We found that protein A from Staphylococcus aureus (SpA) induced Type II interferon production in human PBL. Maximum interferon production was observed 24 hours after treatment with SpA. Moreover, unlike other mitogen interferon inducers, induction of interferon was enhanced by macrophage depletion. Cell fractionation studies showed that the interferon-producing cell was a nonadherent Fc receptor-positive lymphocyte. Studies on the effects of Type II interferon on natural killing (NK) and antibody-dependent cell-mediated cytotoxicity (ADCC) suggest that both NK and ADCC are augmented by Type II IF treatment.